Re: [RFC PATCH v2 2/5] vhost_user: Add frontend command for shmem config

[Stefan Hajnoczi]

On Fri, Jun 28, 2024 at 04:57:07PM +0200, Albert Esteve wrote:
> The frontend can use this command to retrieve
> VIRTIO Shared Memory Regions configuration from
> the backend. The response contains the number of
> shared memory regions, their size, and shmid.
> 
> This is useful when the frontend is unaware of
> specific backend type and configuration,
> for example, in the `vhost-user-device` case.
> 
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  docs/interop/vhost-user.rst       | 31 +++++++++++++++++++++++
>  hw/virtio/vhost-user.c            | 42 +++++++++++++++++++++++++++++++
>  include/hw/virtio/vhost-backend.h |  6 +++++
>  include/hw/virtio/vhost-user.h    |  1 +
>  4 files changed, 80 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index d52ba719d5..51f01d1d84 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -348,6 +348,19 @@ Device state transfer parameters
>    In the future, additional phases might be added e.g. to allow
>    iterative migration while the device is running.
>  
> +VIRTIO Shared Memory Region configuration
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> ++-------------+---------+------------+----+------------+
> +| num regions | padding | mem size 0 | .. | mem size 7 |
> ++-------------+---------+------------+----+------------+
> +
> +:num regions: a 32-bit number of regions
> +
> +:padding: 32-bit
> +
> +:mem size: 64-bit size of VIRTIO Shared Memory Region
> +
>  C structure
>  -----------
>  
> @@ -369,6 +382,10 @@ In QEMU the vhost-user message is implemented with the 
> following struct:
>            VhostUserConfig config;
>            VhostUserVringArea area;
>            VhostUserInflight inflight;
> +          VhostUserShared object;
> +          VhostUserTransferDeviceState transfer_state;
> +          VhostUserMMap mmap;
> +          VhostUserShMemConfig shmem;
>        };
>    } QEMU_PACKED VhostUserMsg;
>  
> @@ -1051,6 +1068,7 @@ Protocol features
>    #define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
>    #define VHOST_USER_PROTOCOL_F_SHARED_OBJECT        18
>    #define VHOST_USER_PROTOCOL_F_DEVICE_STATE         19
> +  #define VHOST_USER_PROTOCOL_F_SHMEM                20
>  
>  Front-end message types
>  -----------------------
> @@ -1725,6 +1743,19 @@ Front-end message types
>    Using this function requires prior negotiation of the
>    ``VHOST_USER_PROTOCOL_F_DEVICE_STATE`` feature.
>  
> +``VHOST_USER_GET_SHMEM_CONFIG``
> +  :id: 44
> +  :equivalent ioctl: N/A
> +  :request payload: N/A
> +  :reply payload: ``struct VhostUserShMemConfig``
> +
> +  When the ``VHOST_USER_PROTOCOL_F_SHMEM`` protocol feature has been
> +  successfully negotiated, this message can be submitted by the front-end
> +  to gather the VIRTIO Shared Memory Region configuration. Back-end will 
> respond
> +  with the number of VIRTIO Shared Memory Regions it requires, and each 
> shared memory
> +  region size in an array. The shared memory IDs are represented by the index
> +  of the array.

Please add:
- The Shared Memory Region size must be a multiple of the page size supported 
by mmap(2).
- The size may be 0 if the region is unused. This can happen when the
  device does not support an optional feature but does support a feature
  that uses a higher shmid.

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